patch 8.0.0878: no completion for :mapclear

Problem:    No completion for :mapclear.
Solution:   Add completion (Nobuhiro Takasaki et al. closes #1943)
diff --git a/src/version.c b/src/version.c
index c85cf1e..de89ce4 100644
--- a/src/version.c
+++ b/src/version.c
@@ -770,6 +770,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    878,
+/**/
     877,
 /**/
     876,